Heat Transfer Equipments

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Brian Howard
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1 Heat Transfer Equipments The transfer of heat to and from process fluid is an essential part of most chemical processes. The word exchanger really applies to all types of equipments in which heat is exchanged. There are many types of heat transfer equipment such: 1-Double pipe exchangers: One of the simplest and cheapest types, it is a concentric pipe arrangement as shown below, it is only used for a small heat transfer area. Fig. (1) Double pipe heat exchanger Hair pin exchangers are another types of double pipe heat exchangers, and it is formed by inserting one or more U-tubes into two pipe section welded to a large flanged, these exchangers are cheaper than shell and tube at very small sizes and can be specified for 7 to 170 m 2 area. 2- Air Cooled Exchangers: It is consist of banks of finned tubes over which air is blown or drawn by fan mounted blown or above the tubes, it is a package unit. Application: air cooled exchangers used if a- Water is expansive or there is shortage in cooling water. b- Process temperature above 65 0 C. c- Climate with high humidity, so air cooling will be cheaper than cooling tower. 1

2 d- It is needs to avoid increasing the existing cooling tower load. Design Information: The tubes used in air coolers are usually finned provide additional surface area. The ratio of finned tube to bare tube 20:1 Standard tube length are used, the tube welded to a header at each end of the exchanger. The height of the tube bank above the ground must be at least half of the tube length to give an inlet velocity equal to the face velocity. The air temperature considerations (in design calculations) the highest temperature that exceeded for 400 h/year. The fan mounted into ways. A-Above the tube racks (Induced draft) see fig.(2) : Advantages: 1. A good air distribution across the tube racks. 2. Less air recirculation. 3. The housing around the fan is similar to chimney effects. Disadvantages: 1. The maintenance across is difficult. B-Below the tube racks (forced draft) see fig.(2) : Advantage: 1. Easier access for maintenance. 2. Reduce power requirements. 3. In winter air recirculation will may offset the effect of lower ambient temperature. 4. Less expensive than induced draft. 2

Disadvantages: 1. Air recirculation may happen during normal operation. Fig. (2) Typical induced draft air-cooled exchanger showing two exchanger section and two fans.

3 Disadvantages: 1. Air recirculation may happen during normal operation. Fig. (2) Typical induced draft air-cooled exchanger showing two exchanger section and two fans. 3- Direct-Contact Heat Exchangers: In direct-contact heat exchangers the hot and cold streams are brought into contact without any separating wall. 1. High rates of heat transfer are achieved. Advantages: 2. The equipment is simple and cheap. 3. It may use for heavily fouling fluids and liquid containing solids. 4. The size of the exchangers is not critical. 3

4 Applications: it is applied when the process stream and coolant are compatible cooling towers, reactor off-gas quenching. Fig. (3) Typical direct-contact cooler (baffle plates) 4

5 4- Shell and tube Exchangers: Advantages: It is the most common type of heat transfer equipment used: 1- The configuration gives a large surface area in small volume. 2- Good mechanical lay-out. 3- Well-established fabrication techniques. 4- Well-established design procedure. 5- Constructed from a wide range of materials. 6- Easily cleaned. Shell and tube consists of a bundle of tubes enclosed in a cylindrical shell. The ends of the tubes are fitted into tube shell, which separate the shell side and tube-side fluids. Baffles are provided in the shell to direct the fluid flow and support the tube. Note: - temperature approach in shell and tube exchangers are between 5-10 ºC. Types of Shell and Tubes: A- Fixed tube shell (TEMA type BEM) see Fig. (4) Advantages: 1- Simple, cheap. 2- May be used for high pressure. 5

6 Disadvantages: 1- Difficult to cleaning. 2- No provision of thermal expansion so the limiting temperature differences 80 0 C (between fluid in shell and tubes). Fig. (4) Fixed tube shell 6

7 B-U-tubes U bundle (TEMA type BEU) see Fig. (5) Advantages: Cheaper than floating head. Disadvantages: 1- The fluid in tube must be relatively clean. 2- It is difficult to replace tubes. Fig. (5) U-tubes U bundle 7

8 C- Floating Head see Fig. (6) 1- Internal floating head (TEMA type AET and AES) Advantages: 1- Very suitable for high temperature differential. 2-The tubes can be ridden from end to end and the bundle removed. 3- Can be used for fouling liquids. Disadvantages: 1- The clearance between the tubes and the shell may allowing fluids to fluids to bypass the tubes. 2- External floating head (TEMA type AEP) Advantages: The same as internal floating head. Disadvantages: 1-Is limiting for 20 bar pressure. 2- Flammable and toxic materials should not used in shell side. 8

9 Fig. (6) Floating Head 9

10 5- Plate Heat Exchangers: Gasketed-Plate Heat Exchangers: It is consists of a stack of closed spaced thin plates clamped together in a frame, a thin gasket seals the plates around their edge. Advantages: 1- Low approach temperature can be used as 1 C. 2- The exchangers more flexible easy to add extra plates. 3- Suitable for highly viscous materials. 4- The temperature correction factor higher than shell and tube. 0 Disadvantages: 1-The limitation of pressure not exceeds 30 bar. 2-The selection of suitable gasket is critical. 3-The maximum operating temperature is limited to about C. 4- Expansion than other types. 5- The maximum flow rates of fluid 2500 m 3 /hr. 10

Fig. (7) Casketed-plate heat exchanger Design Information: The plate surface area 0.3 1.

11 Fig. (7) Casketed-plate heat exchanger Design Information: The plate surface area m 2 The plate width /length ratio 2-3 The plate thickness The gas between plates Material of construction 0.3 3mm 1.5 3mm material and alloys 11

12 6 Fired heaters furnaces and boilers: They are directly heated by the products of combustions of a fuel They are constructed either rectangular or cylindrical steel chamber lined with refractory bricks Tubes are arranged around the wall horizontally or vertically the fluid to be heated flows through the tubes. UDesign Information Tube diameter mm Typical tube velocity's 1-2 m/s The fuel used: - natural gas, fuel oil, off-gases from the process Excess air:- 20% for gaseous fuels 25% for liquid fuel Fig. (8) Fired heater (a) Vertical-cylindrical, all radiant. (b) Verticalcylindrical, helical coil. (c) Vertical-cylindrical with convection section. 12

13 U7- Heat transfer to vessels: A- UJacket vessels:- The commonly used type is consists of an outer cylinder that surrounded part of the vessel. The heating or cooling medium circulates in the annular space between the jacket and the vessel wall. The space between the jacket and vessel wall typically a range from 50 to 300 mm There are other types of jacket such as spirally baffled jacket, half pipe jacket. UJacket selection factors:- 1-Cost 2- heat transfer rate required 3- Pressure Fig. (9) Jacketed vessels. (a) Spirally baffled jacket. (b) Dimple jacket. (c) Half-pipe jacket. (d) Agitation nozzle. 13

14 UR0R = hr0r = hrir = krwr = drir = = Design procedure: The prime objective in the design of an exchange is to determine the surface area required for the specified duty using the temperature differences available. The general equation for heat transfer across a surface is:- Where:- Q = heat transferred per unit time, W. QQ = UU AA ΔΔΔΔRm U = the overall heat transfer coefficient W/m².ºC. Δ TRmR the mean temperature difference (driving force) ºC The overall coefficient is the reciprocal of the overall resistance to heat transfer. where the overall coefficient based on the outside area of the tube, W/m2 C, outside fluid film coefficient, W/m2 C, inside fluid film coefficient, W/m2 C, hrodr = outside dirt coefficient (fouling factor), W/m2 C, hridr = inside dirt coefficient, W/m2 C, thermal conductivity of the tube wall material, W/m C, tube inside diameter, m, dr0r= tube outside diameter, m. 14

15 Overall Heat Transfer Coefficient Typical values for various types of heat exchangers can be found in the following. 1-TEMA Tubular heat Exchanger Manufactures Association 2-Ludwig ę 1965 Applied process Design for chemical and petrochemical plants vol.3 by Gulf publishing company 3- Green.D.W and Perry.R.H 2007 Perry's Chemical Engineers hand book 8 edition McGraw-Hill 4-Sinnott R and Tower G 2009 Chemical Engineering Design 5 th edition by Butterworth-Heinemann. Fouling Factors (Dirt Factors) Fluids (process and service) will foul, the deposited materials normally have allow thermal conductivity and thus normally reduce the overall coefficient. Fouling factors are a heat transfer resistance and not as safety factor in exchanger design. Mean temperature Differences TRlm calculated from the differenc in the fluid temp. At the inlet and outlet & the exchanger. (logarithmic mean) The assumption used:- It is only applicable to sensible heat transfer in co- current or counter current flow. Linear temp enthalpy curve.- Heat capacities & both stream are constant.- There is on phase change.- There is no heat losses- 15

16 FRtR = must have For counter current For shell and tube the flow will be a mixture of co- current counter-current and cross flow. To estimate the true temperature difference FRtR the temperature correction factor. been, use which called shell and tube fluid temperature no.tube and shell passes R = shell side fluid flowrate specific heat = T1 T2 tube side fluid flowrate specific heat t2 t1 S= t2 t1 T1 t1 the temp. efficiency of the exchanger Assumptions to calculate FRt 1- Equal heat transfer area. 2- A constant overall heat transfer coeff. in each pass. 3-The temp of the shell side in any pass is constant across section. 4-there is no leakage of fluid between shell passes. FRt Rwill fall it there is a temp. cross where it is occur if the outlet temp. of cold stream is greater than the outlet temp. of the hot stream. For economic reason FRtR be not less than

17 Shell and Tube Exchangers construction Details Shell and tube consist of a bundle of tubes. enclosed in a cylindrical shell, the ends of the tubes are fitted into tube sheets which separate the shell side and tube side fluids. Tubes in Exchangers Tube used ¼ -2 in recommended 5/8-1 in Because they give a more compact exchangers larger diam. May use to heavily fouling fluids Tube Thickness Depends on internal and external pressure and corrosion allowance. Tube length Actually it is need to know the tube length available in local market. Notes:- Using longer tubes will reduce shell diameter lower cost and better heat transfer rate, but pressure drop will be high Tube Arrangement 1- Equilateral triangular. Pattern 2-Squrare Pattern 3-Rotated Square.Pattern The recommended tube pitches 1.25 times the tube outside diam. Tube Side passes To increase tube side design velocity the tube arranged in parallel and directed the fluid in the tube to flow back(pppppppppppp) usually, To 16 passes 17

18 The recommended velocities in tubes are:- For liquids, 1 to 2 m/s, max 4 m/s Vapor at atmosphere pressure High pressure Vacuum Shell side m/s 5-10 m/s m/s 0.3 to 1 m/s Shells Shell diameter from 10 in t0 6o inch it is preferred to use pipe if available. Shell Thickness Shell are considers as pressure vessels so the thickness may estimate by :- T= PPPP DDDD + c 2ssδ 1.2pppp Where:- T= the minimum thickness required, mm Pi= Internal pressure, N/ mmp Di=Internal Diam., mm S= maximum allowable stress, N/ mmp δ = Welded joint efficiency. C=corrosion allowances. Estimation & heat transfer Coefficient Inside tubes h Ri 2 2 hii dddd kkkk = jjh RRRR pppp^0.33 µ µww Where jh can be obtained from figures 18

19 Viscosity Correction Factor It used only for viscous liquids 1-calculate the coefficient without correction 2-hi (tw-t) = U(T-t) Where:- t=tube side bulk temp mean. T=shell side bulk temp mean. tw=estimated wall temp. 3- Made a trial and error to get the actual tw. Estimation of Heat Transfer Coeff. in shell side h o DD ee = 1.1 dd oo (pp tt dd oo 2 ) Gs= Re = As= Where:- PRtR=tube pitch. dror=tube outside diam. DRsR=shell inside diam. 19

20 P P P Pressure Drop Calculation in tube Side There are two sources for pressure drop 1- friction loss in the tubes. 2- loss due to sudden contraction and expansion flow reversal in the headers. The optimum P For liquid Allowable press. drop viscosity 2 <1 mns/mp 2 P35 kn/mp 1to 10 mns/mp 2 2 P50-70 kn/mp For gases or Vapors. High vac. Medium vac kn/mp 0.1*absolute press Bar 0.5*system gauge pressure above 10 bar 0.1*system gauge pressure prt R=Np m=for laminar flow R for turbulent flow Re 2100 =0.14 Np=No.& tube side passes URtR=tube side velocity.m/s Pressure Drop. Calculation in shell side Ps= 8jf 20

21 CONDENSERS This section covers the design of shell and tube exchangers used as condensers. The construction of a condenser will be similar to other shell and tube exchangers, but with a wider baffle spacing, typically IB = D Four condenser configurations are possible: 1. Horizontal, with condensation in the shell, and the cooling medium in the tubes. 2. Horizontal, with condensation in the tubes. 3. Vertical, with condensation in the shell. 4. Vertical, with condensation in the tubes. Horizontal shell-side and vertical tube-side are the most commonly used types of condenser. A horizontal exchanger with condensation in the tubes is rarely used as a process condenser, but is the usual arrangement for heaters and vaporisers using condensing steam as the heating medium. The normal mechanism for heat transfer in commercial condensers is film wise condensation. Using Kern's method, the mean coefficient for a tube bundle is given by: 21

22 and L = tube length, W c N N t r = total condensate flow, = total number of tubes in the bundle, = average number of tubes in a vertical tube row. Nr can be taken as two-thirds of the number in the central tube row. (h cc ) ss = 0.76kk LL ρρ LL (ρρ LL ρρ vv ) gg μμ LL ΓΓ h 1/3 Steam is frequently used as a heating medium. The film coefficient for condensing steam can be calculated using the methods given in the previous sections; it is customary to assume a typical, conservative, value for design purposes. For air-free steam a coefficient of 8000 W/m 2 C (1500 Btu/h ft 2 F) can be used. A pure, saturated, vapour will condense at a fixed temperature, at constant pressure. For an isothermal process such as this, the simple logarithmic mean temperature difference can be used in the equation 12.1; no correction factor for multiple passes is needed. The logarithmic mean temperature difference will be given by: where T sat = saturation temperature of the vapor, t1 = inlet coolant temperature, 22

23 t 2 = outlet coolant. Reboilers and vaporizers Three principal types of reboiler are used: 1. Forced circulation, Fig. 10: in which the fluid is pumped through the exchanger, and the vapour formed is separated in the base of the column. When used as a vaporiser a disengagement vessel will have to be provided. Fig. (10) forced-circulation reboiler 2. Thermosyphon, natural circulation, Fig. 11: vertical exchangers with vaporization in the tubes, or horizontal exchangers with vaporisation in the shell. The liquid circulation through the exchanger is maintained by the difference in density between the two-phase mixture of vapour and liquid in the exchanger and the single-phase liquid in the base of the column. As with the forced-circulation type, a disengagement vessel will be needed if this type is used as a vaporiser. 3. Kettle type, Fig. 12: in which boiling takes place on tubes immersed in a pool of liquid; there is no circulation of liquid through the 23

24 exchanger. This type is also, more correctly, called a submerged bundle reboiler. Choice of type The choice of the best type of reboiler or vaporiser for a given duty will depend on the following factors: 1. The nature of the process fluid, particularly its viscosity and propensity to fouling, 2. The operating pressure: vacuum or pressure. 3. The equipment layout, particularly the headroom available. Forced-circulation reboilers are especially suitable for handling viscous and heavily fouling process fluids. The major disadvantage of this type is that a pump is required and the pumping cost will be high. Fig. (11) horizontal thermosyphon reboiler 24

25 Fig. (12) kettle reboiler Fig. (13) internal reboiler Thermosyphon reboilers are the most economical type for most applications, but are not suitable for high viscosity fluids or high vacuum operation. A 25

26 disadvantage of this type is that the column base must be elevated to provide the hydrostatic head required for the therrnosyphon effect. Kettle reboilers have lower heat-transfer coefficients than the other types, as there is no liquid circulation. They are not suitable for fouling materials, and have a high residence time. They will generally be more expensive than an equivalent therrnosyphon type as a larger shell is needed, but if the duty is such that the bundle can be installed in the column base, the cost will be competitive with the other types. Design of kettle reboilers 1. The tube arrangement, triangular or square pitch, will not have a significant effect on the heat-transfer coefficient. 2. A tube pitch of between 1.5 to 2.0 times the tube outside diameter should be used to avoid vapour blanketing. Long thin bundles will be more efficient than short fat bundles. 3. The shell should be sized to give adequate space for the disengagement of the vapour and liquid. The shell diameter required will depend on the heat flux. can be used as a guide: 2 Heat flux W/m Shell dia./bundle dia. 25, to ,000 to 40, to , to 2.0 The freeboard between the liquid level and shell should be at least 0.25 m. 26

27 Mean temperature differences When the fluid being vaporised is a single component and the heating medium is steam (or another condensing vapour), both shell and tubes side processes will be isothermal and the mean temperature difference will be simply the difference between the saturation temperatures. If one side is not isothermal the logarithmic mean temperature difference should be used. If the temperature varies on both sides, the logarithmic temperature difference must be corrected for departures from true cross- or counter-current flow. 27

How is the heat transfer?

How is the heat transfer? As we discussed early in the first chapter that heat can transfer through materials and the surrounding medium whenever temperature gradient exists until thermal equilibrium is